Low frequency modulation expander



1 TOS/GNAL March 4, 1941. BURNSHjE 2,233,769

LOW FREQUENCY MODULATION EXPANDER Filed April 18, 1959 r AE 4400014750) E L 7 CARRIERS cs i f) G f u/W v 2 NETWORK m 0 :://z II .E MPH ER PUSH-PULL AMPLIFIER sol/RE I NV EN TOR. 00M 6. BURNS/DE ATTORNEY.

Patented Mar. 4, 1941 UNITED STATES PATENT OFFICE Don G. Burnside, East Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 18, 1939, Serial No. 268,464

Claims.

My present invention relates generally to audio frequency tone control circuits, and more particularly to a novel method of securing automatic bass compensation in an audio frequency trans- 5 mission network.

It is known that the human ear is relatively insensitive to sounds of low'frequency when the sound intensity is of low value. Whenever it becomes necessary to reduce the volume level of reproduced sound, it sounds to the listeners ear as though there is a serious deficiency in the reproduction of bass notes. This effect arises even though the modulation frequency voltage amplifier be capable of passing the various audio frequency voltage components without altering their relative amplitudes. As is well known, many devices have been employed to augment the bass response of reproduced sound so that the psychological effect of true tone balance is secured.

Now I have devised a novel method of, and means for, automatically securing bass compensationin an audio network; the method taking advantage of the fact that in most broadcast transmitters the low frequency audio signals modulate'the carrier wave to a higher degree than do the high frequency audio signals.

It may be stated to'be one of the main objects of my present invention to arrange a pair of modulated carrier energy rectifiers in opposition so that they produce no direct current output with applied unmodulated carrier of constant or varying amplitude, but give a direct current output dependent on the modulation of the carrier.

Another important object of my invention is to provide opposed signal rectifiers in a modulated carrier energy detector network, the detector providing audio voltage for a subsequent audio network and. direct current voltage whose magnitude is dependent on the degree of modulation of the carrier.

Another object of this invention is to provide in combination with a diode signal detector feeding audio voltage to an audio amplifier, a second diode detector arranged in opposition to the first detector thereby to provide a direct current voltage which increases in magnitude, and in a positive polarity sense, with the degree of modulation of the signal carrier by the low audio frequencies, and the direct current voltage being applied to the audio amplifier in a gain-increasing sense whereby bass compensation is secured.

The novel-features which I'believe-tobe characteristic ofmy invention are set'forth' inparticularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have in- .dicated diagrammatically severalcircuit organizations whereby my invention may be carried into effect.

In the drawing- Fig. 1 illustrates a simple detector network em- 10 bodying the invention,

Fig. 2 shows a detector-audio amplifier arrangement embodying theinvention.

Referring now to the accompanying drawing, and specifically to Fig. 1,'ther.e are shown apair of diodes l andZ having a common carrier input source. The latter may be the output circuit of any pro-detector network of a radio receiver, and the resonant circuit 3 is coupled to the carrier source and is tuned to the operating carrier frequency. Let it be assumed that upon circuit 3 is impressed audio modulated carrier energy, and that the carrieramplitude is maintained substantially uniform at the circuit 3 by any well known type of automatic volume control circuit. 'The diode I has its anode 4connected to cathode 5 through a path including circuit '3 in series with a load resistor 6, a condenser 1 being shunted across the resistor. The diode 2 has its anode 8 connected to cathode 9 through a series path comprising the carrier choke coil l0 and load'resistor H, and condenser 12 is connectedacross the resistor. The junction of coil l0 and resistor ll is grounded, and carrier energy is impressed on anode 8 by the condenser l3 connected between the high potential end of circuit 3 and anode 8. Current indicator devices may be included in series with each diode load resistor to indicate current flow in each rectifier. The audio I network, utilizing the audio voltage output of the demodulator, is shown as connected between ground and the anode end of resistor 6. The diodes may be housed in a single tube envelope if desired.

The resistors 6 and I I have equal resistive magnitudes, and both condensers I and I2'have low impedances to carrier frequency current. Hence for carrier only, the product of the current through resistor'fi and resistance of the latter is equal-to the product of the current through resistor ll and resistance of the latter. There- 'forethe direct currentvoltages, for an unmodulated carrier, across the loa'd resistors Bandll are equaliin valueand opposite in polarity. It

will then be seen that the direct current voltage between terminals A remains unchanged. If, now, the carrier energy applied to the diodes be modulated the direct current voltage across resistor 6 is but slightly altered, since condenser is small (of the order of 100 micromicrofarads (mmf.). The same modulated carrier applied to diode 2 causes condenser l2 to charge to the peaks of the modulated carrier, because condenser |2 is larger as compared to condenser Condenser l2 may have a magnitude of about 1 mid. (microfarads). Accordingly, the direct current voltage across resistor is increased, reaching twice that across resistor 6 with 100% modulation. The direct current voltage at terminals A becomes increasingly positive as a result, since one terminal of A is at ground and the other is connected to the cathode end of resistor Audio voltage, also, appears at A by virtue of demodulation by diode I; diode 2 causes no productionof audio voltage because. of the larger value of condenser I2. Condenser I3 permits only carrier voltage to be applied to diode 2, and coil l0 prevents loss of carrier voltage. The device operates to provide equal voltages across resistors 6 and H with carrier only, regardless of. its amplitude. The direct current voltage across resistor 6 is the carrier peak voltage times the rectification efiiciency, as it is across With the carrier modulated the direct current voltage across II is more nearly the peak of the envelope times the rectification efficiency. The voltage across resistor II is thus over and above that across resistor 6.

The direct current voltage and audio voltage developed at A can be used in any audio amplifier network. Mor specifically, the direct current voltage is employed to increase the gain of the audio amplifier. When the volume leve1 of the sound output of the audio amplifier is reduced to a point where normally a reduced bass response seems to occur, the eifect of the direct current voltage at A will be to augment the audio gain and thereby compensate for the lowered bass response. In Fig. 2 is shown a specific circuit embodying the bass response augmenting network. It is assumed that the network is embodied in a superheterodyne receiver, and that input circuit I4 is the intermediate frequency output circuit of the last intermediate frequency amplifier.

The circuit I4 is coupled to the demodulator, or detector, input circuit |5 which is similarly resonated to the operating intermediate frequency. The diode I6 has its anode connected to its cathode through a path including circuit l5 and load resistor carrier by-pass condenser l8 shunting resistor Diode l9 has its anode connected to its cathode through a path comprising choke coil 20 and load resistor 2|; carrier by-pass condenser 22 shunting the resistor 2|. The condenser 23, of low impedance to the intermediate frequency currents, couples the anode of diode I9 to th input circuit l5.

The cathodes of the diodes l6 and I9 areconneoted together, and a single tube envelope embodying the electrodes of a pair of diodes may be used. The anode end of resistor 2| is returned to ground by a lead 24. The lead 24 is connected to point 25 on the voltage supply resistor 26. An intermediate point 21 of resistor 26 is established at ground potentiaLand point 25 may be a point on the resistorwhich is negative with respect to ground. It is to be understood that the resistor 26 may be connected across the usual power supply network of a commercial broadcast radio receiver.

The audio voltage developed across resistor I1 is impressed upon the control grid 28 of tube 29.

The cathode of tube 29 is connected to ground through a path which includes a resistor 30, and the signa1 grid 28 is connected to an intermediate point 3| on the cathode resistor 3|) through a path which includes the grid leak resistor 32. The audio voltage connection to grid 28 includes the audio coupling condenser 33, and adjustable tap 34 provides control over the amplitude of the audio voltage impressed upon signal grid 28. In other words tap 34 provides the manual Volume control device of the system. It will be understood that resistors I'! and 2| are of equal magnitude, and that whil condensers l8 and 22 are bypass condensers for the intermediate frequency carrier, the magnitude of condenser 22 exceeds that of condenser I8. In other words audio voltage is developed across load resistor due to rectification by diode l6, while no audio voltage is developed across resistor 2| due to the fact that the magnitude of the capacitance of condenser 22 is considerably larger than that of condenser l8. Condenser 22 can have a value of 1 mid. However, there is developed across resistor 2| a, direct current voltage whose magnitude is proportional to the degree of modulation of the intermediate frequency carrier, and this is caused by the fact that the modulated carrier applied to diode |9 through condenser 23 causes condenser 22 to charge to the peaks of the modulated carrier. The direct current voltage developed across resistor 2| reaches twice that across resistor IT with modulation.

The plate of audio tube 29 is connected to a proper positive potential point on. resistor 26 through a load resistor 40. The tube 29 functions as a phase changing amplifier tube, the audio voltage output of tube 29 being impressed upon a pair of tubes 4| and 42 arranged in push-pull connection. Each of the push-pull amplifier tubes may be of the SL7 type. Tube 4| includes a cathode and an output electrode, as well as five grid electrodes. The first, or signal, grid 43 is connected to a desired point on load resistor 40 through a path which includes the audio coupling condenser 44 and the adjustable tap 45. The signal grid 46 of tube 42 is connected to a desired point on the cathode load resistor 30 through apath which includes audio coupling condenser 41 and the adjustable tap 48.

In order to provide normal negative bias, for

,the signal grids ,43 and 46, a resistor 50 connects the two grids and the mid-point of the resistor is connected by lead 5| to a point 52 ;on' voltage supply resistor 26 which is negative, withresp'ect to ground point 21. Since the cathodes of tubes 4| and 42 are established at ground potential,

. it will be seen that the connection; to point- 52 provides the normal negative bias-for the signal grids 43 and 46 thus permitting ,operation of tubes 4| and 42 as audio amplifiers. -The plate elec trodes of. tubes 4| and 42 are connected in common to the opposite ends of the primary winding of the audio output transformer T, :and the midpoint of the primary, winding may beconneoted to a desired positive potentia1po-int on supply resistor 25. It is to-be understood that the secondary winding of transformer Twill be connected to any desired type of audioreproducer device. 11-" f The third grid,'with 'respectto' the cathode; of tube 4|. is denoted by numeral .60yand it'isrconnected by lead 6| to the anode end of load resistor 11, the lead including a filter resistor 62. Similarly, the third grid 10 of tube 42 is connected by lead H to the filter resistor 62. The grids 60 and 10 function as the gain control'electrodes of tubes and 42 respectivelyand they are both connected to the anode end of resistor I! so that they will be unaffected by unmodulated carrier energy impressed on diodes l6 and l 9. For modulated intermediate frequency carrier energy the grids will be biased in a positive polarity sense by the direct current voltage developed across resistor 2l, it being noted that effectively the grids 60 and 10 are connected to the cathode end of resistor 2|. Further it will be noted that normal negative bias is applied to grids 60 and 10 by virtue of the connection of these grids to point 25, this connection being through resistors l1 and 2| andlead 24. Point 25 is negative with respect to point 52, and, therefore, there is a normal negative bias applied to grids 60 and Ill such that the mutual conductance of each tube is at a relatively low value forunmodulated carrier energy.

The gain control electrodes of each of tubes 4| and 42 are located between a pair of positive screen grids, and these pairs of screen grids may be connected to a common positive potential point onthe. resistor 26, as is well known to those skilled in the art. Furthermore, each of the suppressor grids of the tubes 4| and 42 may be established at ground potential. The direct current voltage connections BI and H provide the automatic bass compensation. The audio voltage applied to the signal grids 43 and 46 are of suitable phase for push-pull operation, since the audio voltage for grid 43 is derived from the plate load resistor of tub 2 9, while the voltage for grid 46 is derived from the cathode load resistor 30. When the tap 34 is adjusted for impression of audio voltage of maximum magnitude on grid' 28, then it will be understood that there is little need for the bass compensation action since to the listeners ear there will not appear any deficiency of the intensity of reproduction of the bass notes. However, when the tap 34 is adjusted for impression of audio voltage of minimum amplitude on grid 28 then the automatic bass compensation is effective.

The bass compensationis effected by virtue of the fact that the low audio frequencies modulate the intermediate frequency carrier to a much greater extent thanthe high audio frequencies. Hence, there is developed across resistor 2| 2. direct current voltage whose magnitude is proportional to the degree of modulation of the intermediate frequency carrier and, therefore, the magnitude of the direct current voltage developed across resistor 2| exceeds the value of the direct current voltage developed across resistor ll. Because of this fact a direct current voltage is applied to grids 60 and 10 which is positive in polarity. This positive voltage acts to reduce the high negative bias applied from point 25, and the gain of each of tubes 4| and 42 is increased. It will, therefore, be seen that for adjustment of the audio network to reduce volume level, there occurs the compensating increase in the gain of each audio amplifiers 4| and 42. This increase in gain will be suflicient to compensate for the apparent impairment of the reproduction of the bass notes.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth inthe appended laims.

What I claim is:

1. "In a system for detecting audio modulated carrier waves which includes means for rectifying a portion of the waves to produce audio modulation voltage and a unidirectional voltage whose value is independent of the modulation degree of the carrier, means for rectifying a second portion of the waves to produce a second uni-directional voltage whose magnitude is dependent on the degree of modulation of the carrier, means combining the two voltages in polarity opposition, means utilizing the audio modulation voltage output of said first rectifying means, and means utilizing the said resultant combined voltage to control the said audio utilizing means.

2. In a system for receiving audio-modulated carrier energy which includes means fordetecting a portion of the energy to produce audio voltage and a direct current voltage whose magnitude is dependent solely on the carrier amplitude, means for detecting a second portion of the energy to produce a direct current voltage whose magnitude is dependent on both the carrier amplitude and the degree of modulation of the carrier, means combining the two direct voltages to produce a control voltage whose value increases with the increase of the degree of modulation of the carrier, means for amplifying said audio voltage and means for expanding the gain of the amplifying means in response to increase in magnitude of said control voltage.

3. In a system of receiving audio-modulated carrier energy which includes mean for detecting a portion of the energy to produce audio voltage and a direct current voltage whose magnitude is dependent solely on the carrier amplitude, means for detecting a second portion of the energy to produce a direct current voltage whose magnitude is dependent on both the carrier amplitude and the degree of modulation of the carrier, means combining the two direct voltages to produce a control voltage whose value increases with the increase of the degree of modulation of the carrier, means for amplifying the audio voltage, and means for controlling the amplification of the audio voltage with said control voltage.

4. In a system of detecting audio modulated carrier waves which includes means for rectifying a portion of the waves to produce a unidirectional voltage whose value is independent of the modulation degree of the carrier, means for rectifying a second portion of the waves to produce a second uni-directional voltage whose magnitude is dependent on the degree of modulation of the carrier, means for combining the two voltages in polarity opposition, means for deriving an audio voltage from solely the said first portion of carrier waves, means transmitting the' audio voltage through an audio network, and means increasing the transmission efiiciency through said audio network in response to said resultant voltage.

5. In combination, in amodulated carrier receiving system, a pair of detectors, each detector including a load resistor, one of said load resistors having means operatively associated therewith whereby an audio voltage is developed across the resistor, means operatively associated with the other load resistor whereby a direct current voltage is. developed across the resistor which is proportional to the degree of modulation of a carrier, an audio amplifier including means for impressing thereon the said audio voltage, means for regulating. the magnitude of said impressed audio voltage, and additional means for increasing the gain of said audio amplifier in accordance with an increase in the magnitude of the aforesaid direct current voltage.

6. In combination, in a modulated carrier receiving system, a pair of detectors, each detector including a load resistor, one of said load resistors having means operatively associated therewith whereby an audio voltage is developed across the resistor, means operatively associated with the other load resistor whereby a direct current voltage is developed across the resistor which is proportional to the degree of modulation of a carrier, an audio amplifier including means for impressing thereon the said audio voltage, means for regulating the magnitude of said impressed audio voltage, and additional means for controlling the gain of said audio amplifier with the aforesaid direct current voltage, said audio amplifier including a signal grid having a connection to said load resistor across which the audio voltage is developed, and a second connection from the audio amplifier to said resistor across which is developed said direct current voltage whereby the latter voltage is transmitted over said second connection.

'7. In combination, in a modulated carrier receiving system, a pair of diode detectors, each detector including a load resistor, one of said load resistors having means operatively associated therewith whereby an audio voltage is developed across the resistor, means operatively tional means for controlling the gain of said audio amplifier with the aforesaid direct current voltage, said audio amplifier including a signal grid and again control grid, an audio voltage connection between the signal grid and said resistor which develops said audio voltage, and an independent connection between said gain control electrode and said resistor which develops said'direct current voltage.

'1: 8., In an audio-modulated carrier wave receiving system, a pair of rectifiers having a common carrier wave input circuit, means operatively associated with said rectifiers for deriving from rectified modulated carrierwave current a unidirectional vo1tage,'whose magnitude is a function of the degree of modulation of the carrier, and .an audio modulation voltage, amplifier means utilizing said modulation voltage, and means responsive to increase in magnitude of said unidirectional voltage for controlling said utilizing means in a gain-increasing sense.

In an audio-modulated carrier wave re ceiving system, a pair of rectifiers having a common carrier wave input circuit, means operatively associated with said rectifiers for deriving Irom rectified modulated carrier wave current a unidirectional voltage, whose magnitude is a function of the degree of modulation of the carrier, and an audio modulation voltage, means utilizing said modulation voltage, and means responsive to said unidirectional voltage for controlling said utilizing means, and said utilizing means comprising a pair of audio amplifier tubes arranged in push-pull relation, means for adjusting the magnitude of the audio voltage impressed upon said amplifier tubes, and said controlling means being connected to said amplifier tubes to increase the gain thereof in response to an increase in magnitude of said unidirectional volt- DON G. BURNSIDE. 

